Circuit breaker



July 1, 1958 J. F. MARQUIS CIRCUIT BREAKER Filed May 28, 1957 INVENTOR.

United States Patent ClRQUlT BREAKER John F. Marquis, St. Charies, Ill.,assignor to Littelfuse Incorporated, Des Plaines, 111., a corporation ofIllinois Application May 28, 1957, Serial No. 662,026 Claims. (Cl.200-113) This invention relates to circuit breakers which utilizesnap-acting bimetallic elements as the actuating means therefor.

In circuit breakers of this type, the current being controlled usuallypasses through a bimetallic element which has been initially deformed orprestressed into a dished shape. The bimetallic element includessuperimposed strips of metal having substantially different thermalcoefiicients of expansion. Under normal temperature conditions, themetal having the highest thermal coeflicient of expansion is on theconcave side of the deformed bimetallic element. Heat is generated inthe bimetallic element by the current passing therethrough, and, as thetemperature of the element progressively increases, the high expandingside thereof tends to flatten out and eventually reaches a snappingpoint when the temperature reaches a point indicating a current flowrequiring a circuit interruption. The bimetallic element carries amovable contact which, in the normal dished condition of the bimetallicelement, makes contact with a stationary contact. When the controltemperature is reached, the

bimetallic element snaps into an oppositely dished condition, and themovable contact is pulled away from the stationary contact to therebybreak the circuit. With the cessation of current flow, the temperatureof the bimetallic element drops, and, when the reverse snappingtemperature is reached, the bimetallic element snaps back into a circuitclosing position if the circuit breaker is of the usual self-resettingtype. If the circuit breaker is of the manual resetting type, then amanual reset control must be operated to return the bimetallic elementto a circuit closing position.

Circuit breakers of the type being described may be utilized in avariety of ways. For example, they may be used as a flasher switch forinterrupting a circuit leading to the lights on an advertising sign oron an automobile signal light, or they may be used as a protectivedevice for limting excessive current flow in a circuit. Where it is usedas a protective device, it may be desirable to provide a significanttime lag between the time that the excessive current begins to flow inthe circuit and the time the circuit breaker opens. This time lag is ofimportance where the circuit breaker is used in situations where thecurrent flow may normally exceed only momentarily a given control limit,such as occurs when an electric motor is turned on, and where a circuitinterruption is, therefore, unnecessary except where the excessivecurrent is prolonged beyond a safe time interval.

Among the objects of the present invention are to provide an improvedcircuit breaker of the bimetallic type which provides an appreciabletime lag between the initiation or a current which, it maintained, wouldcause the switch to snap, and the instant the bimetallic elementactually snaps into a circuit opening position, to provide such acircuit breaker which requires a high ambient temperature to operate thesame whereby the circuit breaker may be utilized in environments havingrelatively high ambient temperatures, such as in motor housings, and

(:4 to provide such a switch which is rugged in construction, foolproofin operation and which may be inexpensively manufactured.

In accordance with the most preferred form of the invention, the switchblade of the circuit breaker is made of a dished bimetallic element ofpreferably elongated rectangular shape and havin a wide H-shaped cutoutor slot centered therein, so that the opposite legs of the H areparallel to the short sides of the bimetallic element. The legs of theH-shaped cutout or slot are spaced apart a substantial amount so thatthe metal material between the legs of the slot contributessubstantially to the snap-acting characteristics of the bimetallicelement. Moreover, the legs of the H slot extend to points contiguous tobut spaced from the margins of the bimetallic element, to provide anumber of spaced-apart points where the current path through thebimetallic element are substantially narrowed. Most preferably, theratio of the distances from the margin of the bimetallic element to theweb of the H-slot, on the one hand, and to the end of the legs of theH-slot, on the other hand, is at least in the neighborhood of two toone, so that the heat developed by current flowing between the ends ofthe bimetallic element is concentrated at the ends of the slot legs. Theresistance per unit path length at the ends of the slot legs will betwice that of the metal between the legs where the above-mentioned ratiois two to one. Since, for a given current, the heat loss through aresistance in which the current is flowing is proportional to the valueof the resistance, the heat developed per unit path length in the metalat the slot legs will, in the above example, be two times that in themetal between the legs.

An electrical contact is secured to one end of the bimetallic element ata point outside of one of the slot legs. A connecting terminal means issecured at a corresponding position at the other end of the bimetallicelement. This terminal means may be a stationary ter minal post suitablyconnected to the bimetallic element, or, it could be another contact. Acontact mounting base made of insulating material is provided whichsupports one or more stationary contacts which, under normal circuitoperating conditions, makes engagement with the movable contact orcontacts carried by the bimetallic element. With this construction, twoparallel current paths are provided between the terminal or contactpoints on the bimetallic element, which paths include the substantiallynarrowed path portions afforded by the H-slot. With the wide l-l-slotcentered in the dished bimetallic element, the material between the legsof the wide I slot becomes a dominating influence in the control of thesnap action characteristics of the bimetallic element.

For zero or low current values, the temperature of the bimetallicelement may be predominantly affected by the ambient temperature. Thisambient temperature uniformly heats the entire bimetallic elementincluding the substantial metal portions between the widely spaced H-slot legs, which, under normal ambient temperature conditions, providesappreciable stiffness requiring a high ambient temperature condition toraise the bimetallic element to the snapping temperature. If the currentflowing through the bimetallic element should then momentarily increaseto an abnormally high value, as for example when a motor is initiallyturned on, the narrow current path points at the ends of the slot legswill immediately heat up to a substantial temperature which wouldordinarily cause the bimetallic element to snap into a circuit openingposition, once the metal portions between the slot legs are heated to ahigh temperature also. Since the latter metal portions between the slotlegs have a relatively low resistance, only a relatively small amount ofheat is developed by the current flowing therein. The heat concentratedat the ends of the legs will gradually heat said metal portions betweenthe legs of the H-slot. When the current flowing through the narrowcurrent path points exceeds the control limit for the'delay period, themetal between the slot legs will reach a temperature which will causethe bimetallic element to snap into a circuit opening position. Thus,this arrangement provides a time delay circuit breaker which has a highambient temperature operating point by the simple expedient of a wideH-shaped slot with relatively r long legs providing asubstantialconcentration of heat at the ends of the legs of the slot.

Other objects, advantages and features of the invention will becomeapparent upon making reference to the specification to follow, theclaims and the drawings wherein:

Fig. 1 is an exploded view ofthe parts making up the most preferred formof the invention; 7

Fig. 2 is a central longitudinal vertical sectional View through thecircuit breaker of the invention;

Fig. 3 is a horizontal sectional view of the circuit breaker, takenalong section line 33 in Fig. 2;

Fig. 4 is a horizontal sectional view of the circuit breaker, takenalong section line 4-4 in Fig. 2; and

Fig. 5 is an enlarged transverse vertical sectional view of the circuitbreaker, taken along section line 5-5 in Fig. 2.

One form of circuit breaker constructed in accordance with the presentinvention is generally designated at 10, and includes a generallyrectangular housing 12 having a base portion 14 made of insulatingmaterial and a metal cover 16, and an elongated, rectangular, dishedbimetallic snap-acting element 18 mounted within the housing 12. A metalterminal post 20 and a stationary contact 22 are secured in spaced-apartrelation upon the base portion 14 of the housing by terminal studs 24and 26 and nuts 28 and 30. The base portion 14 has recesses 32 and 34which receive inwardly directed nibs 36 and 33 formed in the coverbottom to hold the cover and base portion of the housing together.

The dish-shaped bimetallic element 18 is supported at one end from thetop of the terminal post 20 by welding, riveting or otherwise suitablysecuring the bimetallic element thereto, with the normally concave sideof the bimetallic element facing the base portion 14 of the housing. Inthe embodiment illustrated in the draw ings, the terminal post is formedwith a reduced end it) (Fig. l), which is passed through a hole 42' atone end of the bimetallic element and then turned over the top of thebimetallic element to lock it to the post. The bimetallic element ismade of two elongated rectangular superimposed metal strips 50 and 52,secured together as by welding and deformed in any suitable way into asegmental spherical shape. Under normal temperature conditions, themetal strip 52 on the convex of the bimetallic strip has a relativelylow thermal coefficient of expansion and the metal strip Stl on theconcave side thereof has a relatively high thermal coeflicient ofexpansion. For example, the metal strip 52 may be made or" lnvar or thelike and the other metal strip 59 may be formed of brass, copper,nickel, chromium or iron alloys, or'the like. The distal end of thebimetallic element carries a contact 53 riveted or otherwise suitablysecured to the bimetallic element on the concave or high expanding sideof the bimetallic element. Under normal temperature conditions, thecontact 53 is in engagement with the stationary contact 22.

In accordance with the most preferred form of the invention, thebimetallic element 18 is provided with a wide generally H-shaped cutoutor slot 54 centered therein between the terminal post 2% and the movablecon tact 523. Both the legs 54'54' and the web 54" of the H-slot arerelatively narrow so that a minimum amount of material is removed fromthe bimetallic element. Moreover, the legs 54-54 of the slot areappreciably spaced apart so that the metal portions between the legshave a predominating efiect on the snapacting characteristics of thebimetallic element. This is because the innermost or most depressedportion of a dished shaped bimetallic element normally has the greatesteffect over the stiffness of the element. The ends of the slot legsextend to points contiguous to but spaced from the long sides of thebimetallic element, so as to provide narrow current paths at the endsthereof, identified as points P1, P2, P3 and P4, relative to the widthsof the current paths between the slot legs. The ratio of the distance d2from the margin at each long side of the bimetallic'strip to the H-slotweb 54", to the distance d1 from the corresponding margin to the end ofeach adjacent slot leg, is preferably at least in the neighborhood of2/1 so that the resistance ratio thereat is at least 1/2 and the heatratio thereat is at least 1/2 (the heat developed per unit length isproportional to the value of the resistance for a given current). Withthis construction, it can be seen that the heat developed by the flow ofcurrentv between the movable contact 53 and the terminal post 20 isconcentrated at the narrow current path points P1, P2, P3 and P4.Moreover, since the formation of the H-slot has removed only arelatively small amount of the material from the center of thebimetallic element, the rigidityof the same is aflected only to a smalldegree, so that the element may be made to operate at a relatively highambient temperature snapping level.

The bimetallic element is designed so that at abnormal or low currentlevels, the heat developed by the fiow of current through the bimetallicelement is insufficient to raise the temperature of the snap-actingcontrol portions of the bimetallic element, that is the region of thebimetallic element between the slot legs, to the snapping point. Whenthe current initially exceeds the control value for which the bimetallicelement is designed, the temperature at the narrow path points P1, P2,P3 and P4 rises to a point which would eventually cause the bimetallicelement to snap into an oppositely dished condition if the currentremained at this level. Since the heat developed by the flow of currentthrough the much wider path portion between the slot legs is relativelysmall, the temperature of this portion of the bimetallic element willnot be very high at the beginning of the high current condition.However, since heat flows from points of high temperature to points oflower temperature, the snap-acting control portion of the bimetallicelement between the slot legs will gradually increase until it reaches apoint where the bimetallic element will snap to the oppositely dishedshape shown in dotted lines in Fig. 2. When this occurs, the distal endof the bimetallic element is raised to bring the movable contact 53 to aposition spaced substantially from the stationary contact 22, therebyopening the circuit in which the circuit breaker is connected. When thecurrent is interrupted, the temperature of the bimetallic element dropsuntil'a point is reached where the element snaps back into' a circuitclosing position shown in solid lines in Fig. 2. Of course, if theambient element increases to the snapping point, this will also causethe bimetallic element to snap into a circuit opening position, eventhough the current passing through the bimetallic element has notreached a value which itself would cause the eventual snapping of thebimetallic element. No appreciable time delay in the action of thebimetallic element would occur here because the control temperature thenalmost immediately exists over the entire bimetallic element.

As above explained, although the embodiment of the invention illustratedis of the self-resetting type, the principles of thepresent inventionmay be applied equally to a switch which requires a manual reset.

The present invention thus provides a'n exceedingly simple andeconomical bimetallic, delay acting circuit breaker adapted foroperation at very high ambient temperature conditions.

It should be understood that numerous modifications may be made of themost preferred form of the invention above described without deviatingfrom the broader aspects of the invention.

I claim as my invention:

1. A circuit breaker comprising a thin, flexible, snapacting bimetallicswitch blade element formed by two superimposed and secured togethermetal strips having substantially diiferent thermal coefficients ofexpansion, said bimetallic element having a first dished shape below agiven control temperature, with the higher expanding metal on theconcave side thereof, and being adapted to snap into an oppositelydished shape above said temperature, said bimetallic element having acontact and terminal means spaced from said contact, said bimetallicelement having an H-shaped electrical insulating portion in the centralregion thereof between said contact and terminal means, the oppositelegs of the l i-shaped insulating portion extending contiguous to butbeing spaced from at least one margin thereof and forming at least onecurrent path between said contact and terminal means having pointslocated at the ends of the legs of the H where the current path narrowsto a degree that the resistance per unit path length is at least in theneighborhood of twice that of the adjacent central portion of thebimetallic element between the legs of the H, so that the heat developedper unit path length at the narrow path points is at least in theneighborhood of twice that developed in said adjacent central portion ofsaid bimetallic element, said latter portion of said bimetallic elementcontributing substantially to the snap-action characteristics of thebimetallic element and receiving suficient heat from said narrow pathpoints under excessive current flow that the snapping temperaturethereof is reached after a time delay from the instant of time theexcessive current begins to flow, and a stationary contact supportedexternally of said bimetallic element and positioned to make firmcontact with said contact on said bimetallic element when the latter isin said first dished shape and which is separated from said lattercontact when said bimetallic element is in its oppositely dished shape.

2. A circuit breaker comprising a thin, flexible, snapacting bimetallicswitch blade element formed by two superimposed and secured togethermetal strips having substantially difierent thermal coefiicients ofexpansion, said bimetallic element having a first dished shape below agiven control temperature, with the higher expanding metal on theconcave side thereof, and being adapted to snap into an oppositelydished shape above said temperature, said bimetallic element having acontact adjacent to one side thereof and terminal means on the oppositeside thereof, said bimetallic element having a wide H- shaped cut-outportion in the central region the cot be tween said contact and terminalmeans, the opposite legs of the H extending contiguous to but beingspaced from the opposite margins thereof, the cutout portion formingparallel current paths between said movable contact and terminal meanseach having points located at the ends of the legs of the H where thecurrent path narrows to a degree that the resistance per unit pathlength is at least in the neighborhood of twice that of the adjacentcentral portion of the bimetallic element between the legs of the H sothat the heat developed per unit path length at the narrow path pointsis at least in the neighborhood of twice that developed in said adjacentcentral portion of said bimetallic element, said latter portion of saidhimetallic element contributing substantially to the snap actioncharacteristics of the bimetallic element and receiving sufiicient heatfrom said narrow path points under excessive current flow that thesnapping temperature thereof is reached after a time delay from theinstant of time the excessive current begins to flow, and a stationarycontact supported externally of said bimetallic 6 element and positionedto make firm contact with said contact on said bimetallic element whenthe latter is in said first dished shape and which is separated fromsaid latter contact when said bimetallic element is in its oppositelydished shape.

3. A circuit breaker comprising a thin, flexible, elongated, rectangularsnap-acting bimetallic switch blade element formed by two superimposedand secured together metal strips having substantially difierent thermalcoefiicients of expansion, said bimetallic element having a first dishedshape below a given control temperature, with the higher expanding metalon the concave side thereof, and being adapted to snap into anoppositely dished shape above said temperature, said bimetallic elementhaving a contact at one end thereof and terminal means at the oppositeend thereof, said bimetallic element having a wide H-shaped cutoutportion in the central region thereof, the opposite legs of the H beinggenerally parallel to the short sides of said bimetallic element, thecutout portion forming parallel paths between said movable contact andterminal means each having points located at the ends of the legs of the5-! where the current path narrows to a degree that the resistance perunit path length is at least in the neighborhood of twice that of theadjacent central portion of the bimetallic element between the legs ofthe H, so that the heat developed per unit path at the narrow pathpoints is at least in the neighborhood of twice that developed in saidadjacent central portion of said bimetallic element, said latter portionof said bimetallic element contributing substantially to the snap actioncharacteristics of the bimetallic element and receiving sufiicient heatfrom said narrow path points under excessive current flow that thesnapping temperature thereof is reached after a time delay from theinstant of time the excessive current begins to flow, and a stationarycontact supported externally of said bimetallic element and positionedto make firm contact with said contact on said bimetallic element whenthe latter is in said first dished shape and which is separated fromsaid latter contact when said bimetallic element is in its oppositelydished shape.

4. A circuit breaker comprising a thin, flexible, snapacting bimetallicswitch blade element formed by two superimposed and secured togethermetal strips having substantially different thermal coefiicients ofexpansion, said bimetallic element having a first dished shape below agiven control temperature, with the higher expanding metal on theconcave side thereof, and being adapted to snap into an oppositelydished shape above said temperature, said bimetallic element having acontact and terminal means thereon for making electrical connection tothe bimetallic element, said bimetallic element having an H-shapedelectrical insulating portion in the central re" gions thereof, whereinthe opposite legs of the H extend '3 contiguous to but are spaced fromopposite margins thereof, the insulating portion being located betweensaid movable contact and terminal means forming parallel current pathsbetween said movable contact and terminal means each having pointslocated at the ends of the legs of the H where the current path narrowsto a degree that the resistance per unit path length is at least in theneighborhood of twice that of the adjacent central portion of thebimetallic element between the legs of the H, so that the heat developedper unit path length at the narrow path points is at least in theneighborhood of twice that developed in said adjacent central portion ofsaid bimetallic element, said latter portion of said bimetallic elementcontributing substantially to the snap action characteristics of thebimetallic element and receiving sufiicient heat from said narrow pathpoints under excessive current flow that the snapping temperaturethereof is reached after a time delay from the instant of time theexcessive current begins to flow, and a stationary contact supportedexternally of said bimetallic element and positioned to make firmcontact with said contact on said bimetallic ele- 7 a ment when thelatter is in said first dished shape and which is separated from saidlatter contact when said bimetallic element is in its oppositely dishedshape.

5. A circuit breaker comprising a thin, flexible, snapacting bimetallicswitch blade element formed by two superimposed and secured togethermetal strips having substantially difilerent thermal coeflicients ofexpansion, said bimetallic element having a first dished shape below agiven control temperature, with the higher expanding metal on theconcave side thereof, and being adapted to snap into an oppositelydished shape above said temperature, said bimetallic element having acontact and terminal means thereon for making electrical connection tothe bimetallic element, said bimetallic element having an H-shapedcut-out portion in the central regions thereof, wherein the oppositelegs of the H extend contiguous to but are spaced from opposite marginsthereof, the cut-out portion being located between said movable contactand terminal means and forming parallel current paths between saidmovable contact and'terminal means each having points located at theends of the legs of the H where the current path narrows to a degreethat the resistance per unit path length is at least in the neighborhoodof twice that of the adjacent central portion of the bimetallic elementbetween the legs of the H, so that the heat developed per unit pathlength at the narrow path points is at least in the neighborhood oftwice that developed in said adjacent central portion of said bimetallicelement, said latter portion of said bimetallic element contributingsubstantially to the snap action characteristics of the bimetallicelement and receiving suflicient heat from said narrow path points underexcessive current flow that the snapping temperature thereof is reachedafter a time delay from the instant of time the excessive current beginsto flow, and a stationary contact supported externally of saidbimetallic element and positioned to make firm contact with said contacton said bimetallic element when the latter is in said first dished shapeand which is separated from said latter contact when said bimetallicelement is in its oppositely dished shape.

References Cited in the file of this patent UNITED STATES PATENTS2,266,537 Elmer Dec. 16, 1941 2,707,216 Farison Apr. 26, 1955 2,709,732Davis May 31, 1955

